1. Field of the Invention
The present invention relates to an optical submarine repeater, and more particularly to a technique for improving radiation characteristics and voltage-resistant characteristics of an optical submarine repeater.
2. Description of the Related Art
In an optical submarine cable transmission system adopting an optical fiber cable whose transmission capacity is larger than that of a coaxial cable, optical submarine repeaters are provided at suitable intervals in order to amplify attenuated optical signals and prevent degradation of transmitted signals. In such an optical submarine repeater, it is desired to improve the radiation characteristics in response to an increase in heating value due to recent high-density mounting or the like. Further, it is also necessary to improve the voltage-resistant characteristics.
A conventional optical submarine repeater is composed generally of a substantially cylindrical pressure-resistant housing for protecting the interior of the repeater from the seawater pressure and a plurality of circuit units including feeding circuit units and amplifying circuit units. The pressure-resistant housing has a pair of cable retaining portions for introducing optical submarine cables at the opposite ends.
The circuit units are accommodated in a substantially cylindrical metal case closed at its opposite ends through a connecting bar for positioning and fixing the circuit units. The metal case is accommodated in a substantially cylindrical PE cylinder formed of polyethylene resin or the like, closed at its opposite ends, in order to improve the voltage-resistant characteristics. The PE cylinder is accommodated in the pressure-resistant housing through a radiating/cushioning member for radiation and cushioning purposes. The PE cylinder accommodating the metal case is sandwiched by a pair of substantially annular radiating/cushioning members, thereby being fixed to the pressure-resistant housing. The optical submarine cable is introduced from an end of the cable retaining portion of the pressure-resistant housing. An optical fiber cable and a feeder cable are passed through through-holes formed through an end plate of the PE cylinder and an end plate of the metal case, and are connected to the corresponding circuit units.
As an amplifying system for such an optical submarine repeater, a 3R repeating system has conventionally been adopted. In the 3R repeating system, an electrical signal converted from an optical signal is subjected to Reshaping, Retiming, and Regeneration, and is then reconverted into an optical signal. Recently, however, an optical direct amplification system using an erbium-doped fiber or the like has increasingly been adopted.
In association with the shift to such an optical direct amplification system, high-density mounting has proceeded to result in an increase in heating value per unit volume in the optical submarine repeater as mentioned above. Further, a distance between neighboring repeaters becomes shorter, so that in the case where the repeaters are laid on a shallow seabed, the radiation of heat from the repeaters is reduced because of a relatively high seawater temperature at the shallow seabed. In a WDM (Wavelength Division Multiplexing) system under consideration for future adoption, a further increase in heating value is expected. Accordingly, sufficient radiation measures must be taken. While the optical submarine repeater is operated by the supply of given electric power through a feeder cable, there is a possibility that a large potential difference (e.g., several kilovolts) between the pressure-resistant housing and the circuit units contained therein may be generated in the event of abnormality in a feeding path. Accordingly, the voltage-resistant characteristics must also be improved.